1. Field of the Invention
The present invention relates generally to valves, and more particularly to valves used for freeze sealing carbon and low alloy steel pipes with cryogenic fluids.
2. Description of the Prior Art
During pipe maintenance, it is often necessary to create a plug inside a pipe to prevent fluid from flowing through the pipe. This plug may be formed by wrapping copper tubing around the outside of the pipe and circulating very cold fluid through the tubing. This lowers the pipe's temperature and freezes any fluid located inside the pipe. The frozen fluid creates a solid plug, preventing any fluid flow until the pipe's temperature is raised and the plug has thawed.
At very low temperatures these pipes become subject to brittle fracture. As a result, the pipe's temperature must be carefully controlled. This temperature is controlled by drawing liquid nitrogen from a storage tank or dewar at -321.degree. F. and warming the nitrogen to about -35.degree. F. The nitrogen (now a gas) is then circulated through copper tubing.
For optimum protection of the pipes, the nitrogen's temperature must be maintained as close as possible to -35.degree. F. The nitrogen's temperature may be controlled by carefully opening and closing a valve to regulate the nitrogen's mass flow rate. The degree of flow rate control is directly proportional to the precision of the valve's adjustment. Current valves are not able to accurately and precisely control nitrogen's flow rate because of three problems.
First, current valves generally employ either a needle or a disk to control fluid flow. However, neither design is suitable for creating freeze plugs. Valves employing a disk generally operate as either fully open or fully closed. Such designs are effective for total fluid isolation, but lack the requisite precision for accurately adjusting fluid flow rates. In contrast, needle valves offer more precise flow control, but do not positively stop or prevent fluid flow.
U.S. Pat. Nos. 4,705,062, by Baker and 4,923,173, by Szmaszek et al. disclose typical needle and disc valves. As discussed above, the efficiency of these valves is limited by internal movement of the valve parts and lack of positive flow isolation.
U.S. Pat. No. 3,598,145, by Wolfson, discloses a valve designed to rapidly respond to small changes in internal fluid system pressure but operates as either fully open or fully closed. This valve, like the disk valves, is inapplicable to the present application because the nitrogen temperature control method requires a valve allowing precise fluid flow control rather than operating as fully opened or closed.
The second problem limiting the application of current valve designs arises because valves currently used for precise flow rate control are not designed for use in low, i.e., cryogenic, environments. The extreme temperatures necessary for forming freeze plugs exaggerate the valves'inherent mechanical looseness leading to further loss of flow control. In addition, such extreme temperatures may also cause the valves'moveable parts to bind or structurally fail.
U.S. Pat. No. 3,511,475, by Pfau, discloses a valve which attempts to provide precise fluid flow adjustment by using a rubber-like sealing element. However, this valve is not designed for use in the harsh environment present at cryogenic temperatures.
In contrast, valves which are currently used in cryogenic environments lack adequately precise fluid control. U.S. Pat. Nos. 3,902,695, by Worwetz; 3,905,424, by Elwood et al., and 4,956,975, by Gustfson, all disclose valves designed for use in cryogenic environments. All three valves lack the necessary precision for use in the present application.
Finally, current valves also have inherent mechanical looseness. This looseness permits the internal valve assembly to float, i.e. move laterally and vertically, reducing the accuracy of flow rate control.
This movement may be controlled by securing the valve's internal members to the valve stem. For example, U.S. Pat. No. 4,766,927, by Conatser, discloses a valve designed to regulate fluid flow, volume or pressure of fluid pouring into a pump. This valve employs a split washer to secure a valve part to a valve stem. However, the split washer merely prevents the spring support from sliding off the valve stem but does not secure or stabilize the valve's interior members.